www/grafana8: Update to 8.1.6

ReleaseNotes: https://grafana.com/blog/2021/10/05/grafana-7.5.11-and-8.1.6-released-with-critical-security-fix/
PR: 258962
Security: CVE-2021-39226
(cherry picked from commit 725c5eb1df6a47cad84a95b05ee2460868a93096)